The invention relates to a method and an apparatus for the production of a longitudinal seam welded hollow profile from a sheet-metal blank with parallel longitudinal edges.
In the automobile industry, welded open profiles are being increasingly replaced by thin-walled hollow profiles, for which longitudinal seam welded tubes are the starting stock. Having small wall thicknesses, these components are designed to achieve minimum weight and maximum utilisation of material.
To ensure the technical functionality of such space-frame structures, it is necessary to have control over the production process for the components, right down to the final shaping step.
The profile-forming process in which the tubular elements that are used as hollow profiles are formed is an important part of the process sequence. Custom profiles known as “tailored tubes” are generally formed in a discontinuous operation, that is to say, from ready cut blanks. Tailored tubes are tubular elements that are made from sheet-metal sections with material properties that are adapted to the stresses and demands that arise during practical use or during the shaping process.
Various options are available for forming sheet-metal blanks into finished welded profiles. However, most solutions use separate work stations for forming and welding (DE 44 32 674 C1).
An apparatus of the abovementioned type that enables longitudinal seam welded tubular elements to be produced on one station is known for example from German patent specification No. 966 111. In this apparatus, the sheet-metal blank is formed into a slit tube and welded. For this purpose, a flat sheet is held between two tool halves that are mutually movable on a tool carrier and disposed in mirror-image relationship to one another, each with a half-shell recess that defines the external contour of the tube that is to be produced. The longitudinal edges of the sheet-metal blank are oriented parallel to the recesses in the tool halves such that the tool halves receive the edges that are adjacent to them as they are driven together and, viewed in cross-section, move the edges towards one another along a circular arc. While the tool halves come together, the sheet metal blank is held by holders positioned at the ends of the centre line of the sheet-metal blank adjacent to the two short edges of the sheet-metal blank. This ensures that when the tool halves are driven together, the sheet-metal blank is uniformly moved along the contour defined by the recesses in the tool halves until its longitudinal edges meet one another at the apex of the curve. The slit tube thereby formed is held in this position for its longitudinal edges to be welded together. To make the slit area of the slit tube accessible, the upper end portions of the tool halves are then hinged open. Thereafter, the adjacent longitudinal edges in the area of the slit are welded together.
A major disadvantage of the procedure known from German patent specification No. 966 111 is that the processed metal sheet must have a certain minimum stiffness. This is the only way to ensure that the metal sheet curves uniformly to form a slit tube when the tool halves are driven together. This method cannot be used to form thin metal sheets into a tube with a precisely predetermined cross-sectional shape, because they uncontrollably form edges and folds during shaping which render the resulting tubular element useless. Welding a slit tube formed by the method known from German patent specification No. 966 111 will also result in unwanted distortion if the metal sheet is so slender that it cannot withstand the forces that unavoidably act on it during welding.
One attempt to overcome the disadvantages of apparatuses of the above-described type is known from WO 99/67037. The procedure known from this specification combines forming and welding on one workstation. A tool is used which, in the manner of German patent specification No. 966 111, comprises two tool halves that can be moved towards one another on a tool carrier, each with a cylindrical half-shell recess. In addition, an internal mandrel half is associated with each of the recesses of the tool halves, positioned within each recess and rigidly fastened to each of the tool halves, with a gap left between its peripheral surface and the inside surface of the recess. This forms an annular space in the region of the recess in each tool half.
To form the tubular element, the thin metal sheet to be shaped is placed between the two tool halves such that when the tool halves are subsequently moved towards one another, its longitudinal edges are guided into the annular space formed in each of the tool halves. As the tool halves continue to come together, the longitudinal edges are pushed further up the longitudinal gap and the metal sheet is curved into a slit tube. The support that is afforded in the region of the annular space to the sheet-metal blank on both the inside and outside of the metal sheet ensures that no unwanted fold or edge formation occurs.
This forming process, which is known as “rolling”, is complete when the longitudinal edges meet at the apex of the resulting slit tube. After the edge joint has been rounded off with a roller, this slit tube can be welded together along the slit without having to be moved to another apparatus.
Although the method known from WO 99/67037 enables precisely-shaped tubular elements to be produced, in practice it has a number of disadvantages. For example, the divided construction of the internal mandrel requires a high degree of manufacturing precision for the tools that are used in the known apparatus. Moreover, the machine is complex in design, because it requires complicated pull-off units to remove the finished welded tubular hollow profile from the divided internal mandrel, and special hold-down means during the forming step. In addition, particularly when thin metal sheets are used, there is still the risk of edge stresses causing undesirable distortion in the area of the welded seam during welding.
With the above-described prior art as its starting point, the object of the invention is to provide a method and an apparatus for producing precisely-formed hollow profiles having a reduced requirement for equipment and shorter production times.
As regards the method, this object is achieved by a method for the production of a longitudinal seam welded hollow profile from a sheet-metal blank with defined longitudinal edges                wherein the sheet-metal blank is placed between two tool parts lying in a starting position in which they are apart, each provided with a recess determining the external shape of at least one section of the hollow profile that is to be produced,        wherein a mandrel, whose external shape defines the internal shape of the hollow profile that is to be produced, is then positioned relative to the recess of one tool part such that a gap is formed between its peripheral surface and the inside surface of the recess, the thickness thereof being slightly greater than the thickness of the sheet-metal blank that is to be shaped,        wherein the metal sheet is held in the area of the half that is distant from the mandrel,        wherein the tool part adjacent to the mandrel is moved together with the mandrel, with the gap intact, in the direction of the longitudinal edge of the sheet-metal blank that is distant from the mandrel such that the sheet-metal blank runs into the gap along its longitudinal edge that is adjacent to the mandrel until it reaches an insertion limit,        wherein the mandrel is positioned relative to the recess of the second tool part such that a gap is formed between the external surface of the mandrel and the inside surface of the said recess, the thickness thereof being slightly greater than the thickness of the sheet-metal blank that is to be shaped,        wherein the tool part now adjacent to the mandrel is moved together with the mandrel, with the gap intact, in the direction of the longitudinal edge of the sheet-metal blank that is distant from the mandrel such that the sheet-metal blank runs into the gap along its longitudinal edge that is adjacent to the mandrel until it reaches an insertion limit and a slit profile that surrounds the mandrel about its peripheral surfaces is formed from the sheet-metal blank,        wherein the mandrel is moved out from the interior of the slit profile,        wherein the longitudinal edges of the sheet-metal blank that define the slit are welded together, the slit profile being supported by the tool parts, and        wherein the tool parts are moved to their starting position in order for the finished welded hollow profile to be removed from the tool parts.        
In contrast to the prior art described in WO 99/67037, the invention uses at least two steps to form the sheet-metal blank into a slit profile that is subsequently welded to produce the finished hollow profile. For this purpose a mandrel is used which cooperates with the first tool part in the first step, and with the second tool part in the second step, such as to form a gap with the recess of each tool part into which a portion of the sheet-metal blank is guided.
Shaping the sheet-metal blank in at least two steps means that a mandrel that is a self-contained component and can therefore be moved by a simple control device can be used to support the sheet-metal blank while the slit profile is being created. This in itself considerably reduces the equipment needed to produce the hollow profile.
Moreover, forming the sheet-metal blank into the slit profile in at least two steps also considerably improves the reliability with which this shaping takes place. Only one half of the sheet-metal blank is shaped at any one time, while the other half is securely held. This results in improved stability of the sheet-metal blank during shaping, so that consequently the overall shaping process can be faster. The risk of disruption caused by the metal sheet being bent out of shape inside the gap is thus minimized. This means that the sheet-metal blank can be inserted into the gap at a greater speed. Moreover, the resources needed to separate the mandrel and the hollow profile are also reduced because the mandrel can be moved as a whole, so that its movements too can be rapid. Consequently, even though the method of the invention is carried out in two steps, production time as a whole is reduced by comparison with production times attainable in the prior art and precision is improved at the same time.
As regards the apparatus, the above object is accordingly achieved by an apparatus for producing a longitudinal seam welded hollow profile from a sheet-metal blank with defined longitudinal edges, provided with at least two tool parts that can be moved together into a forming position from a starting position in which they are apart, and that each have a recess determining the external shape of at least one section of the hollow profile that is to be produced, and with a welding device for welding together the longitudinal edges of the sheet-metal blank after it has been formed into a slit profile, wherein, in accordance with the invention, there are provided                a mandrel carried by a control device that moves it from a first position in which it is positioned in the recess of a first tool part with a gap between the inside surface of the recess of the said tool part and the peripheral surface of the mandrel, into a second position in which it is positioned in the recess of the second tool part with a gap between the inside surface of the recess of said tool part and the peripheral surface of the mandrel, and        a holding device for holding the sheet-metal blank in the region of one of its longitudinal edges.        
A particularly simple and rapid method of removing the mandrel from the hollow profile that is to be produced consists, in an advantageous embodiment of the invention, in moving the tool parts, together with the portions of the sheet-metal blanks lying in their recesses, sufficiently far apart for the mandrel to be moved through the enlarged opening between the two longitudinal edges of the slit profile.
The dimensional stability of hollow profiles produced in accordance with the invention can be improved by pushing the tool parts together, once the mandrel has been moved out of the slit profile, until the edge joint is closed or its width has reached a minimum. In this manner, variations in shape and dimensions can also be minimised in the region of the edge joint and the weld that is to be made there. Moreover, minimizing the slit width makes welding easier. The longitudinal edges along the slit of the slit profile that is preformed from the sheet-metal blank can be particularly easily and reliably brought together if the desired width of the slit is determined by a tongue held in the slit. The equipment required for this purpose can be minimised if the tongue is carried by the mandrel. Preferably, the tongue is sword-shaped and extends along the length of the slit of the slit profile.
The precision with which a hollow profile produced in accordance with the invention is formed can be additionally improved by using at least two steps to close the tool parts together in order to set up the edge joint of the slit profile that is preformed from the sheet-metal blank. Each step can be used to size the slit profile, so that the shape of the slit profile is ever more closely matched at each stage of the forming process to the required final shape of the finished hollow profile. This multistep forming process can be assisted by providing the tongue with at least one shoulder, and in particular with two shoulders of different thickness. Where two shoulders of different thickness are present, the first can determine the desired width of the slit in the first step, and the second can determine the desired width of the slit in the second step of the closing movement.
After welding, an internal high-pressure forming process can be used for further shaping. With the tool parts still closed, internal high-pressure forming enables additional shape elements to be introduced into the hollow profile. Recesses can be provided in the tool parts for this purpose, adjustments to the predetermined shape being made in this manner.
In another variant of the method of the invention that minimizes the equipment that is required, the mandrel is lowered onto the sheet-metal blank so that it exerts a pressure that securely holds the sheet-metal blank on an underlying surface.